It is well-known that lead is usually produced from galena by a thermal way i.e., by means of roasting processes.
This production method involves a large number of problems, above all of the environmental type, owing to the emission of considerable amounts of dusts containing lead and sulfur dioxide into the atmosphere.
Another problem which begins to constitute a considerable burden which damages the budgets of the industry of the production of lead by extraction thereof from galena by the thermal way is the consequent production of sulfuric acid, which leads often to financial burdens and to general disadvantages.
Owing to these reasons, most experts in this sector agree upon expecting that in a near future the production of lead by the hydrometallurgical process will become competitive with the present production by thermal way.
In the relevant technical literature a very large number of papers exist, reporting about the studies and researches aiming at developing a hydrometallurgical process for producing lead from galena.
Suffice it to say that the first investigations carried out by Bequerel and Marchese in order to obtain sulfur and lead from galena by electrolysis date back to the second half of the nineteenth century. The thermodynamic and kinematic parameters of the reaction of dissolution of lead in an electrolyte and production of elemental sulfur were subsequently investigated into greater details by a large number of researchers.
The oxidating means which is by far the most studied one, is, still to-day, ferric chloride.
With this reactant, during the past years the two most advanced processes for the hydrometallurgical processing of galena were developed, and precisely the Minimet Penarroya process and the U.S. Bureau of Mines (USBM) process.
In both of these methods, a leaching of galena in an aqueous solution of ferric chloride with NaCl is carried out first, then the sulfur-containing residue is filtered off, and the so obtained lead chloride is electrolysed.
It is in this latter operation that the two processes are different from each other, because according to the Minimet process, the solution is submitted to electrolysis after being purified, with spongy lead being obtained, whilst according to the USBM process PbCl.sub.2 is crystallized and is then submitted to electrolysis in a bath of molten chlorides.
However, also the use of hydrometallurgy which chloride leaching is affected by drawbacks which derive from the specific characteristics of chloride ion, i.e.; * low solubility of lead chloride in water; hence, the need of adding such salts as NaCl or, in general, alkali and alkali-earth metal chlorides, which, as well-known, increase the solubility of the metals by forming the Cl.sub.4 - complex ion.
The complex chloride, although is beneficial to the solubility of lead, causes the dissolving of nobler metals such as Bi, Ag and Cu, so that these latter metals are subdivided between the residue and the solution in a difficultly foreseeable way. PA2 In general, from the smelting of the lead sponge obtained from the electrolysis, a lead with a purity of 99.99% is obtained, unless the electrolyte is submitted to a preliminary, laborious purification.
* Also in case such a contrivance is adopted, the leaching solutions can dissolve not more than 25-30 g of Pb** per liter. Therefore a "galena/solution" ratio equal or lower than 1:20 required. PA1 * The electrolysis of lead chloride in an aqueous solution does not yield a compact deposit; on the contrary, lead is recovered as an incoherent sponge. The electrolytic cell must have a very complex structure in order to collect the product which falls to the bottom, and remains impregnated with electrolyte. PA1 * the smelting of the lead sponge, owing to its high oxidability, is a delicate operation, to be carried out under a flux (NaOH), and causes the production of at least 5% of oxidation slags. PA1 (a) Galena is leached with an acidic aqueous solution of ferric fluoborate, with ferrous fluoborate, lead fluoborate and elemental sulfur being formed according to the reaction: EQU 2 Fe(BF.sub.4).sub.3 +PbS.fwdarw.2 Fe(BF.sub.4).sub.2 +Pb(BF.sub.4).sub.2 +S PA1 (b) the solid residue, composed by elemental sulfur and galena gangue is filtered off; PA1 (c) the solution of ferrous fluoborate and lead fluoborate coming from the (a) step is sent to a diaphragm electrolytic cell, wherein pure lead is deposited at the cathode and at the anode ferrous ion is oxidated to ferric ion; PA1 (d) the solution of ferric fluoborate regenerated at the anode in said (c) step is recycled to said (a) step of galena leaching.
The electrolysis of lead chloride in a molten electrolytic bath is much more complex, is not safe from the environmental viewpoint, consumes a larger amount of energy and the produced lead does not have a purity of 99.99%.